-Trials and Tribblelations
-In a Mirror, Darkly
All these are considered among the best episodes in their respective series. None of the 4 spinoffs have this kind of homage from each other;
The spinoffs are called spinoffs;
Kirk is mentioned in dialogue in the other series (except ENT of course -- but they went up to consider bring Shatner for their 6th season); Spock and Sarek were in TNG;
McCoy appeared in the TNG pilot; the next episode (Naked Now) was a remake of one of TOS, complete with Picard looking in awe at "Kirk's ship" in a computer screen.
TNG wasn't let spawn a movie series unless Shatner was in it, despite the absurdity of the plot that made it so.
TNG does not even has a proper name: its simply The Next
Generation (weekly reminding Kirk's was the first)
The 2009 reboot is TOS. Made millions.